1. Field of the Invention
This invention relates to an apparatus for seismic prospection of the subsoil by vibration.
2. Description of Prior Art
A vibrating source is used to emit a signal into the ground in a known manner, the spectrum of this signal covering the range of signals which are usually transmitted by the subsoil. A signal of this type, termed a pilot signal, may be, for example a vibration of variable frequency, linear or non-linear as a function of time, between two extreme frequencies.
Seismic pick-ups or geophones are positioned at different points on the surface of the ground and, by comparing the signal received by these pick-ups, termed the seismic signal, with the pilot signal, the ground response is obtained which provides useful indications about the nature thereof.
This comparison is generally effected by an cross correlation between the pilot signal and each of the seismic signals.
Thus, if the pilot signal is denoted P(t) and the seismic signal which is received is denoted S(t), the cross correlation function of these two signals is as follows: EQU I(t)= P(x.).S(t+x)dx
In practice, a pilot signal is limited to a duration .theta.p, and the correlation function is only considered over a limited time interval T, termed "thoroughness investigation". Under these conditions, the previous expression becomes the following: EQU I(t)= pP(x).S(t+x)dx,
where t.epsilon.(O,T).
Bearing in mind the considerable dynamic range of the signals, it is desirable for the correlation to be effected in a digital manner. Thus, the pilot signal is memorized in successive pilot samples P.sub.i, i=1 . . . p, p being the number of samples corresponding to the complete duration of the pilot signal, also termed "vibration length". For example, for a vibration length of 30 seconds and a sampling step of 2 milliseconds, p=15,000 samples.
For their part, each of the seismic signals received is numbered in successive samples S.sub.i, where i=1 . . . s, and where s=p+N, N being the number of samples corresponding to the thoroughness of investigation T. For example, when T=5 seconds, N=2,500 samples, s is termed the "acquisition length".
Thus, s=17,500 samples S.sub.i for each seismic signal and in each test which is carried out. It is already possible to note that this number is much higher than that N corresponding to the thoroughness of investigation.
The cross correlation function then becomes the following: ##EQU1## where t=1 . . . N.
Up until the present time, these samples of seismic signals were recorded on magnetic tape, in view of subsequent calculation by a data processing center, being provided with adequately large memory means and calculation means.
In the above-mentioned example, assuming that the work is carried out using 96 seismic channels, it is clear that 17,500.times.96=1,680,000 samples of seismic signals have to be recorded for each test which is carried out in addition to the pilot signal.
Moreover, since the energy which is emitted into the ground during a test is low, several repeated tests are generally carried out while keeping the source and the pick-ups in the same place, and using the same pilot signal. Each new value which is measured is added to the total of the measurements from the previous tests. In practice, since up until the present time, sufficiently powerful calculation means have not been available for carrying out correlation operations on the ground, and since it would have been prohibitive to record the signals obtained during each of the tests, the signals obtained by the same geophone during the same series of tests were cumulated. The correlation operations were then carried out once, off line in time, on the result of the addition.
This method, in which correlation is effected after summation, first of all has the disadvantage of only providing a result from overall values, without considering the variations in the pilot signal from one test to another. A high level of accuracy necessitates a pilot signal which is absolutely constant throughout the measurements. Moreover, it only allows the work to be carried out with an identical pilot signal (to close innaccuracies) for all the measurements.
In contrast, a cross correlation of the signals during each measurement, before summation of the calculated result, allows the work to be carried out with a pilot signal which varies from one measurement to another or, if the same pilot signal is retained, it eliminates deviations discovered from one test to another. Above all, it would present the advantage of providing an immediate result, by calculation in real time, the accuracy of which result would improve as the tests progressed.
The main obstacle which has until now prevented achievement of a solution of this type is that of the size of the memory necessary for making the calculations. Since signals of a long duration are concerned, the number of samples to be registered tends to increase and it becomes very difficult to dimension the memory, numerous versions being necessary depending on the length of the pilot. Up until the present time, these restrictions, have not allowed the provision of a seismic prospecting material which may be easily moved and used on the ground.